Marx, Engels & Theology. By Roland Boer (2017). A Reading Guide:
“The following collection begins with Marx, drawing occasionally on joint works with Engels, before focusing in the last two sections on Engels’s life-long interest in religion.”

The International Humanist-Marxist

Marxism and Religion: a complete and annotated bibliography. By Roland Boer
(December 1, 2012)
“The following collection begins with Marx, drawing occasionally on joint works with Engels, before focusing in the last two sections on Engels’s life-long interest in religion.”

Mikhail Bakunin
(Marxists Internet Archive)

God and the State (1871)
“Bakunin’s most famous work, published in various lengths … this version is the most complete form of the work published hitherto.”

Communism and Religion. With Preobrazhensky. Chapter 11 in The ABC of Communism (1920)
Contents: Why religion and communism are incompatible – Separation of the church from the state – Separation of the school from the church – Struggle with the religious prejudice of the masses.

Church and school in the Soviet Republic (1919)
“One of the instruments for the obscuring of the consciousness of the people is the belief in God and the devil, in good and evil spirits, angels, saints, etc., in short – religion.”

James Connolly
(Marxists Internet Archive)

Labour, nationality and religion (1910)
Contents: The problem stated – The rights of man – The honour of the home – The suicide of a nation – God or mammon – The firebrand or the olive leaf.

V.I. Lenin
(Marxists Internet Archive)

The attitude of the Workers’ Party to Religion (1909)
“We must combat religion – that is the ABC of all materialism, and consequently of Marxism. But Marxism is not a materialism which has stopped at the ABC. Marxism goes further. It says: We must know how to combat religion, and in order to do so we must explain the source of faith and religion among the masses in a materialist way.”

Socialism and Religion (1905)
“Religion must be of no concern to the state, and religious societies must have no connection with governmental authority. Everyone must be absolutely free to profess any religion he pleases, or no religion whatever, i.e., to be an atheist, which every socialist is, as a rule. Discrimination among citizens on account of their religious convictions is wholly intolerable.”

Rosa Luxemburg
(Marxists Internet Archive)

An anti-clerical policy of Socialism (1903)
“That ‘religion is a private affair’ only compels us to be neutral and not to take part in religious questions, when these only refer to intimate convictions and to the conscience … In the name of liberty of conscience, we demand the abolition of all public privileges which believers enjoy to the disadvantage of unbelievers, and we will assail all efforts attempted by the Church to become a dominating power in the State.”

Socialism and the Churches (1905)
“The Social-Democracy in no way fights against religious beliefs. On the contrary, it demands complete freedom of conscience for every individual and the widest possible toleration for every faith and every opinion. But, from the moment when the priests use the pulpit as a means of political struggle against the working classes, the workers must fight against the enemies of their rights and their liberation.”

Socialism and Religion (1907)
“The declaration that religion is a private matter is … an expression of the clearly scientific nature and aim of our struggle, a necessary consequence of our materialist theory of history, and only our materialism is able to give a scientific vindication of this demand.”

Leon Trotsky
(Workers Liberty)

How socialists fight Religion (1924)
“In this article Leon Trotsky discusses the Bolsheviks’ efforts to break the grip of religion on the people of Russia after the workers’ revolution of 1917.”
Extracts from the speech: ‘Leninism and Workers’ Clubs’ (July 17, 1924). – in: Problems of everyday life (Pathfinder, 1973, p. 308-313).